Just a week after her return to the United States Senate after a roughly three month absence, questions continue to swirl around Sen. Dianne Feinstein and her mental capacity to serve in the world's greatest deliberative body.

The 89-year-old Democrat had been recovering from shingles at home in California, and had been absent from the Hill since February.

Her long-awaited return on May 10 not only meant that the Senate Democratic Caucus would be at full attendance -- since both Feinstein and Pennsylvania Sen. John Fetterman had been absent for much of the spring -- but that the one-seat margin Democrats held on the powerful Judiciary Committee would be reconstituted to help advance President Joe Biden's judicial nominations.

Senate Majority Leader Chuck Schumer greeted the frail Feinstein personally upon her return, when she was wheeled into the Capitol for a vote accompanied by staff on and off the floor. Schumer said Feinstein was "exactly where she wants to be, ready to do the things she loves the most -- serving the people of California." First elected to the Senate in 1992, she is the longest-serving woman senator in US history.

But questions quickly sprang up on whether Feinstein, though present, would really be able to resume her demanding job. In a statement released by her office last week, Feinstein said that she is still "experiencing some side effects" from shingles and her doctors have advised her to "work a lighter schedule" as she returned to the Senate. During her arrival at the Capitol for votes, she appeared confused and was heard asking staff, "Where am I going?"

And in an interaction with reporters Tuesday, as reported by the Los Angeles Times and Slate, Feinstein appeared confused by questions about her absence, saying, "I haven't been gone. I've been here, I've been voting. Please, either know or don't know." It is not clear if Feinstein was referring to just the past week since her return or referring to the past several months while she was recovering at home.

CNN has reached out to Feinstein's office to request comment.

Fellow Democrats remain unwilling to discuss Feinstein's ability to serve, saying only they are glad to have a colleague back in the chamber.

"I'm happy she's returned, and that's all I'm going to say about it," Hawaii Sen. Mazie Hirono told CNN.

Illinois Democratic Sen. Dick Durbin, who replaced Feinstein as chairman of the Senate Judiciary Committee, said, "We certainly hope" that Feinstein will be able to serve the remainder of her term in the chamber, but demurred when asked if he is confident that she can serve.

"I can't be the judge of that. But I will tell you that she has to make that decision for herself and her family as to going forward, but we're happy to have her back," he said. "We're monitoring her medical condition almost on a daily basis. Our staff is in touch with her staff."

The top Republican on the panel, Sen. Lindsey Graham of South Carolina, said of Feinstein, "She's a dear friend. As a friend, you can see she's hurting."

Other Republicans echoed that sentiment, wishing Feinstein well, but reluctant to weigh in on her mental acuity.

"I have a lot of respect for Dianne Feinstein. She's been great to work with. She's a great committee member," North Carolina Republican Thom Tillis told CNN's Manu Raju, but said that "I haven't had the chance to speak with her, so I couldn't really comment on it."

"If you just take a look at anybody that spent ten months with a chronic case of shingles, that has a huge impact, I don't care how old you are, but again I just haven't spoken with her," he said.

Texas Republican Sen. John Cornyn said that he is "not qualified to render a diagnosis," but criticized some Democrats for calling on her to resign.

"That seems a little harsh to me. I think that decision ought to be made by Senator Feinstein," he said.

Questions about a Senator's health, and whispers about their fitness to serve, are not new. In the past decades, the median age of the Senate has ticked increasingly upward, with the 118th Congress median age at 65.3 years, according to the Pew Research Center.

The current Senate has multiple members in their eighties, including Feinstein, Senate Minority Leader Mitch McConnell, Vermont Sen. Bernie Sanders, and Iowa Sen. Chuck Grassley. Another 41 Senators are at least sixty-seven years old, the official retirement age in the United States.

In recent years, there have been prolonged absences by members of the Senate, notably Arizona Sen. John McCain, who battled brain cancer and was absent from the Senate almost eight months, but never faced calls from his colleagues to resign his seat.

The late Mississippi Sen. Thad Cochran was also out for several weeks with lingering health issues in the fall of 2017, and faced questions about his metal fitness, appearing frail and pale when he returned. The then-chairman of the influential Senate Appropriations Committee, told reporters that he was fit to serve, and said at the time that he planned to run again in 2020, saying "it's up to the people to decide. I think I am."

But the 79-year old Republican needed to be guided by staffers to a "Senators Only" elevator to find his way to the Senate floor. Cochran resigned from the Senate the following March.

"I regret my health has become an ongoing challenge," Cochran said in a statement announcing the end of a four-decade long career in the Senate. "I intend to fulfill my responsibilities and commitments to the people of Mississippi and the Senate through the completion of the 2018 appropriations cycle, after which I will formally retire from the U.S. Senate."

It is unclear if Feinstein will be given the same gentle off ramp afforded to her colleagues.

On November 2020, Feinstein relented to pressure from other Democrats to give up the chair of the Judiciary Committee. In November 2022, under similar pressure, she announced that she would not want to serve as the Senate Pro Tempore, a high-ranking constitutional position granted to the longest-serving member of the Senate majority. Feinstein also announced the following February that she would not run for re-election in 2024. Her February 16, 2023 votes on the Senate floor would prove to be her last ones for months.

Criticisms for Feinstein's long absence started in earnest in April when fellow California Democrat Rep. Ro Khanna tweeted, "it's time for @SenFeinstein to resign. We need to put the country ahead of personal loyalty." Feinstein's office pushed back on the criticism, arguing that there had not been a significant delay in advancing and confirming judicial nominees.

After Fetterman and McConnell -- who was injured in a fall and spent nearly two weeks in a rehabilitation facility -- returned to the Senate, but Feinstein did not, it prompted more questions about the impasse created by her absence and Feinstein asked Schumer to temporarily replace her on the Judiciary Committee. Schumer proposed that Maryland Sen. Ben Cardin take her spot, but Senate Republicans blocked the effort, saying the move would allow judicial nominees they opposed to advance